Ponds and historic wetlands are essential for preventing excessive phosphorus from reaching downstream waters. These systems remove solids, nutrients, metals and hydrocarbons from stormwater runoff as particles settle to the bottom. However, low oxygen levels in the water can cause phosphorous to be released from bottom sediments. This project examined the effectiveness of strategies to limit that release and reduce negative downstream impacts.
More than 30,000 stormwater ponds and historic wetlands in Minnesota are part of the stormwater treatment system that manages harmful solids and pollutants. One of these pollutants, phosphorus, can move to downstream waters, promoting excessive algae and other plant growth and depleting the water of oxygen. Reducing the amount of phosphorus in ponds—or limiting how much escapes—can help mitigate these negative impacts on water quality.
This project examined pond and historic wetland conditions and evaluated strategies to reduce the amount of phosphorus moving to downstream priority waters. By better understanding the effectiveness of these approaches, state and local stormwater management staff can more effectively allocate resources to limit the negative consequences of excess phosphorous.
“This study evaluated relationships impacting pond performance, such as phosphorous release from sediment and water oxygen levels, and demonstrated how modeling could value-test potential interventions,” said Ross Bintner, Engineering Services Manager,
City of Edina
What Did We Do?
Investigators developed a retention pond modeling program by modifying an existing MnDOT lake model. While the original model assessed lake water quality and simulated thermal stratification and vertical profiles of dissolved oxygen and nutrient concentrations, the revised pond model simulates temperature, dissolved oxygen and phosphorus conditions within the pond and its outflow.
Next, field data was collected from six ponds and historic wetlands to assess water quality, water level, dissolved oxygen, temperature and wind speed. A review of laboratory studies on pond sediments was also conducted to support and verify the updated model.
The pond model was used to evaluate six potential remediation techniques for decreasing the impact of phosphorous: chemical sediment treatment, mechanical aeration, adaptive water level control, wind shelter reduction, watershed pollutant load reduction and lining the pond with clay. Investigators also assessed the cost-effectiveness of each phosphorous mitigation approach.
What Did We Learn?
The study of six ponds and historic wetlands found that many are stratified, or layered, often with low dissolved oxygen and elevated phosphorous concentrations at the bottom. The lack of sufficient oxygen causes phosphorus to release to the surface, allowing it to move to downstream waters. Reducing phosphorus carried in stormwater runoff is essential for improving water quality.
The most cost-effective strategy to achieve this reduction was an intensive street-sweeping program that removes phosphorus-containing materials, such as dirt and vegetation, from roads before runoff carries them into ponds and wetlands.
For agencies already conducting extensive street sweeping, outlet control structures that limit phosphorus transport to downstream waters may be effective, particularly in stratified ponds or historic wetlands with high phosphorus concentrations below the surface layer.
When phosphorus release from sediments is substantial, chemical treatment with alum can be effective in binding phosphorous, keeping it trapped on the bottom and reducing its outflow. However, this treatment typically needs to be repeated every five to 10 years. Adding oxygen through microbubble aeration in the spring can also reduce phosphorus release if sufficient oxygen is maintained, though partial aeration may increase phosphorous export if stratification is not fully addressed.
Adaptive water level control strategies designed for flood mitigation were not effective for reducing phosphorus export.
What’s Next?
Managing phosphorous movement to and from ponds and historic wetlands is critical for protecting water quality. By better understanding the cost-effectiveness of available strategies, state and local stormwater management staff can allocate resources more efficiently to reduce phosphorus pollution and limit harmful algae blooms.
Crossroads 